A group III nitride semiconductor including aluminum (Al) has a direct transition type band structure in the ultraviolet area corresponding to a wavelength of 200 nm to 360 nm, thus it is possible to manufacture the ultraviolet ray emitting device having high efficiency.
The group III nitride semiconductor device is produced by carrying out a crystal growth of the group III nitride semiconductor thin film on the single crystal substrate using a chemical vapor deposition method such as the metal-organic chemical vapor deposition method (MOCVD method), a molecular beam epitaxy method (MBE method), a halide vapor phase epitaxy method (HYPE method) or so. Among these, MOCVD method is capable to control the film thickness in an atomic layer level, and also can obtain relatively high growing speed; thus it is the method which is most widely used method in the industries.
When producing the above mentioned ultraviolet ray emitting device, it is difficult to obtain a substrate having good compatibility of a lattice constant and a heat expansion coefficient with the group III nitride semiconductor crystal including Al. Therefore, generally, the group III nitride semiconductor crystal including Al is formed on a substrate made of different material such as sapphire substrate or silicon carbide substrate or so. Particularly, when the emitting wavelength is the ultraviolet range, the sapphire substrate is widely used from the point of view of the light transmittance.
Also, in the group III nitride semiconductor crystal including Al, two polarity of a group III polarity (for example, when the group III nitride is AlN, Al polarity) and a nitride polarity (N polarity), which are in a back and front relation. In order to obtain a good device characteristic, it is preferable to control the growth condition so that the growth proceeds by exposing the group III polarity plane on the outset surface on said substrate made of different material (the group III polarity growth). The reason for this is, because, a smooth crystal surface in a atomic level can be obtained in case of the group III polarity growth; however on the other hand, in case of the N polarity growth, many polarity inversion domains in which the group III polarity and the N polarity are mixed in the crystal are generated and the surface roughness of the crystal surface becomes significantly deteriorate compared to the group III polarity growth (for example, refer to Nonpatent Literatures 1 and 2). Particularly, this tendency becomes prominent in case of growing the group III nitride single crystal having high content of Al such as aluminum nitride single crystal (AlN).
In regards with AlN, the group III polarity (Al polarity) and the nitrogen polarity (N polarity) will be further explained. The Al polarity has the crystal growth plane at “0001” plane or +C plane, and is defined as those having a unit of a tetrahedral structure having aluminum atom at the center of the tetrahedron (the center of the gravity), and having nitrogen (N) atom at four apex. Also, Al polarity growth refers to the growth while forming such unit. On the contrary to this, the N polarity has a crystal growth plane of “000-1” or -C plane and is defined as those having unit of a tetrahedral structure with N atom at the center of the tetrahedral (the center of the gravity), and having Al atom at four apex. Further, the N polarity growth refers to the growth while forming such unit.
Also, when comparing the characteristics of the physical property of the crystal obtained by these growth, the crystal obtained by the Al polarity growth, the surface smoothness, the chemical resistance and the heat resistance of the “exposed surface” (Al polarity plane) which is the opposite side of the plane joining the sapphire substrate are high; on the other hand, these physical properties of “exposed surface” thereof (N polarity plane) are poor compared to that of Al polarity plane.
As described in above, such difference of the physical properties between each polarity plane is basically the same to the group III nitride single crystal other than AlN, and particularly, to the group III nitride single crystal having high content of AlN; and whether the group III nitride (for example AlN) single crystal grown on the sapphire substrate has carried out the group III polarity growth can be easily determined by using the difference of the above mentioned chemical resistance. That is, the layered body (the layered body formed with the group III nitride single crystal layer on the sapphire substrate) is immersed in the alkaline solution such as potassium hydroxide (KOH) or so, and it can be easily carried out by a simple test of etching test which observes the dissolving condition of the crystal surface after immersing. If the surface of the group III nitride single crystal layer is the group III polarity surface, the etching will not be carried out since it has high resistance against the alkaline solution. On the other hand, if the surface is the N polarity surface, it will be easily etched. Therefore, the group III polarity growth can be determined if the etching mark is not observed by observing the surface of before and after of such test; on the other hand, N polarity growth can be determined if an obvious etching mark is observed.
When growing the group III nitride semiconductor crystal on the sapphire substrate, in order to accomplish the group III polarity growth, it is necessary to intentionally create a condition in which the group III polarity growth can be carried out, that is the condition nearly in a condition that the group III atom is over saturated. As for the specific method to create such condition, before forming the group III nitride single crystal layer on the sapphire substrate, the method to supply only the group III raw material (for example, Al raw material) (for example, refer to Nonpatent Literature 3), or the method to discontinuously supply the group III raw material gas (for example, Al raw material) while without supplying the nitrogen gas during the initial stage of growing the group III nitride single crystal layer (for example, refer to Patent Article 1), or so are proposed.    [Patent Article 1] Japanese Unexamined Patent Publication No. 2009-54782    [Nonpatent Literature 1] Applied Physics Letters vol. 83 (2003) 2811    [Nonpatent Literature 2] Japanese Journal of Applied Physics vol. 44 (2005) L150    [Nonpatent Literature 3] Journal of Crystal Growth 310 (2008) 4932
As the method to accomplish the group III polarity growth, the method to create an over saturated condition of the III group atom by providing the step to supply only the group III raw material (for example Al raw material) is proposed, which is also described in Nonpatent Literature 3 and Patent Article 1.
However, in this case, at the initial stage of growing the group III nitride single crystal, it was necessary to strictly control the supply amount of the group III raw material and the supplying method thereof. Also, according to the Nonpatent Literature 3, in order to allow the further stable group III polarity growth, it was necessary to increase the amount of the group III raw material supplied, however on the other hand, it indicates that the crystal quality tends to decline.
Further, the present inventors has carried out additional experiments to grow the AlN single crystal on the sapphire substrate by following the method disclosed in the Nonpatent Literature 3; then though the Al polarity growth was possible, the substrate after the growth showed slight Al metal color, and the line transmittance at the range of 280 nm or less on above substrate was 60% or less which is low, further the line transmittance against the light of 250 nm was also 60% or less. When producing the ultraviolet emitting device on such AlN layered body, the extraction efficiency of the light released to the outside via AlN layer is significantly lowered, and as a result, the characteristic of the ultraviolet emitting device is expected to be lowered.
Therefore, the first object of the present invention is to propose the novel method that the group III polarity growth can be carried out stably when growing the group III nitride semiconductor crystal such as AlN on the sapphire substrate using the MOCVD method. Also, the second object is to provide the layered body having the group III nitride single crystal layer having a high light transmittance and a good crystal quality on the sapphire substrate by using the above mentioned method.